X-ray detectors are used in electron microscopes (such as scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), and the like), X-ray spectrometers (such as X-ray fluorescence (XRF) spectrometers, particle-induced x-ray emission (PIXE) spectrometers, and the like), and other instruments to analyze the composition of materials. This is schematically illustrated in the accompanying FIG. 1, wherein an energy beam 10 such as an electron beam, proton beam, X-ray beam, gamma ray beam, or the like is directed from an emitter 20 toward a target location on a sample 30 to be analyzed. The emitter 20 can take the form of any appropriate beam emitter (e.g., a cathode in the case of an electron energy beam), and may be associated with beam focusing/steering devices (e.g., magnetic lenses for diverting an electron energy beam as desired), which are not shown in FIG. 1. The atoms of the sample 30 ionize in response to the incident energy beam 10, with electrons within the atoms transitioning between different orbital levels about their nuclei. (Such orbitals are often referred to as the K, L, M, and N shells, and the transitions between shells are often referred to as alpha transitions when they occur between two adjacent shells and beta transitions when they occur between two shells spaced by an intermediate shell.) These electron transitions release energy in the form of X-ray (X-ray photon) emissions from the sample 30, wherein the X-rays have energies and wavelengths which are characteristic of the atoms of the sample 30 from which they were emitted, i.e., they are characteristic of the elemental composition of the sample. Thus, the foregoing instruments can measure and analyze the X-ray energies and/or wavelengths to identify and quantify the elemental composition of the sample 30.
Such instruments also often scan the energy beam 10 across a series of target locations on the sample 30 (and/or the sample 30 is scanned beneath the beam 10) to build a “map” of the sample's composition over its area. Additionally, byproducts from the energy beam 10 can be captured at each target location and can be used to generate an image of the sample 30. For example, where the energy beam 10 is an electron beam, the image may depict the target location's backscattered electrons (electrons from the energy beam 10 which were “reflected” from the target location), or the target location's secondary electrons (electrons knocked out of the specimen by the energy beam 10). In either case, the image provides a visual representation of the target location, though the visual representation may not correspond to the target location's appearance if viewed by the eye under standard light. For example, a backscattered electron image effectively provides a view of the target location's density, and a secondary electron image effectively provides a view of the target location's surface roughness. The generated image(s) can be displayed along with the aforementioned elemental composition data for various regions of the sample 30 depicted in the image(s), providing a user valuable understanding of the nature of the sample 30.
The accuracy of the elemental composition data, and the speed and ease of its collection, depends heavily on how it is generated. In energy dispersive spectrometry (EDS), an EDS detector 40—e.g., a Silicon Drift Detector (SDD), lithium-doped silicon (SiLi) detector, microcalorimeter, photodiode, silicon multi-cathode detector (SMCD), PiN diode, or similar device—detects the X-ray photons emitted from the target location on the sample 30 and their energies (usually measured as voltages), and the relative numbers (counts) of the detected photons and their energies serve as the basis for elemental analysis. The photon counts and energies form a spectrum—more specifically, an energy dispersive spectrum—which is often presented as a histogram wherein the counts from the sample are plotted versus their energy (with an exemplary spectrum of this nature being depicted at 50). The number of counts at specific energies or energy ranges then serve to indicate the aforementioned electron transitions, which can serve as characteristic “fingerprints” of particular elements. Stated differently, a high count or “peak” at a particular energy or energy range can serve to indicate the presence of a particular element within the sample. Thus, by compiling a spectrum containing the counts and energies of the emitted photons and comparing it to reference spectra (spectra generated from substances having known elemental composition), or otherwise identifying the elements giving rise to the peaks, one may obtain information regarding the elements present in the specimen. Additionally, the relative heights of the peaks—i.e., their counts or photon intensities—can provide an indication of the relative quantities of the elements present.
One problem with EDS is that it has somewhat poor resolution, in that the peaks of certain different elements overlap (i.e., certain elements generate peaks which rest at the same energies or energy ranges). As an example, sulfur K-alpha peaks, molybdenum L-alpha peaks, and lead M-alpha peaks display across the same energies or energy ranges. Overlapping peaks can sometimes be “deconvolved” into individual peaks by, for example, comparing a measured EDS spectrum to reference spectra, and determining the reference spectra that most likely combine to result in the measured spectrum. However, because deconvolution can sometimes result in incorrect determination of the elements present and their relative amounts, followup analysis is sometimes needed to determine whether its results are correct.
An alternative to EDS is wavelength dispersive spectroscopy (WDS), which uses Bragg's law of diffraction to “sort” the X-ray photons into their separate wavelengths, and the counts and energies of the photons at the various wavelengths then serve to identify the elements within the sample 30. WDS is also schematically illustrated at the left-hand side of FIG. 1, wherein photons emitted from the target location on the sample 30 are directed at a diffractor 60. Depending on the characteristics of the diffractor 60 and its orientation with respect to the incident photons, photons having a certain wavelength will reflect constructively and can be strongly measured at a WDS detector 70, while photons at other wavelengths will destructively interfere and weakly register at the detector 70. Thus, by changing the angle of the diffractor 60 with respect to the sample 30 (and the angle and/or position of the detector 70 as well, since the angle between the diffractor 60 with respect to the incident photons must generally be equal to the angle between the diffractor 60 and detector 70), and/or by changing the diffractor 60 itself (as by rotating the hexagonal diffractor 60 to expose different faces—which are formed of different crystal diffractors—to the incident X-rays), one may scan through all relevant wavelengths to again generate a spectrum of photon counts/intensities and energies which serves to characterize the elemental composition of the sample 30. The elemental peaks in WDS spectra tend to have significantly higher resolution (i.e., the peaks appear across narrower energy ranges and are “sharper”), with an exemplary WDS spectrum being illustrated in FIG. 1 at 80. This helps to avoid the problem of peak overlap that occurs with EDS. (It is notable that EDS and WDS systems are not conventionally combined in the same device as shown in FIG. 1, and they are shown in combined form here merely for ease of discussion, and also for sake of the discussion of the invention below.) However, WDS has the disadvantage that it is time-consuming: it takes time to tune the WDS spectral collector (i.e., to position the diffractor 60 and detector 70) to scan across a range of wavelengths, and to collect measurements at each resulting wavelength setting. As a result, it is common to tune the WDS spectral collector to collect measurements only at the wavelengths of the emission lines (i.e., peaks) of elements of interest, usually elements whose presence is suspected in the sample 30. This approach can therefore fail to detect certain elements if relevant wavelengths are skipped. WDS also bears difficulties in that it is extremely sensitive to the alignment between the sample 30, diffractor 60, and detector 70: the detector 70 and the target location on the sample 30 must both be located at the same angle with respect to the diffractor 60, and the detector 70 only properly registers photons traveling along the path defined by this relationship. As a result, if the target location on the sample 30 is not located at the same angle with respect to the diffractor 60 as the detector 70, the detector 70 will receive few or no photons, resulting in no or low detector 70 readings. Difficulties in attaining proper alignment can be compounded by drift in the energy beam 10—its axis can change over time—and by the topology of the sample 30 (e.g., differences in surface height, as shown in FIG. 1), factors which lead to variability in the target location on the sample 30. As discussed in U.S. Pat. No. 5,926,522, which usefully contains more details on WDS and EDS (and which is incorporated by reference herein), a collection optic 90 (such as a polycapillary lens/optic) can usefully be situated between the sample 30 and diffractor 60 to capture photons traveling along the aforementioned path, as well as photons traveling along slightly divergent paths, and focus and collimate them so that they travel along the desired path for detection at the detector 70. In effect, the collection optic 90 increases the field of view of the detector 70, thus increasing the number of received photons and allowing reduction in photon collection times. Nevertheless, the system is still sensitive to misalignment.
Owing to the aforementioned difficulties, further increases in accuracy, speed, and ease of WDS (and EDS) would naturally be welcomed.